The antioxidant protein extracellular superoxide dismutase (EC-SOD) encompasses a C-terminal region that mediates interactions with a number of ligands in the extracellular matrix (ECM). This ECM-binding region can be removed by limited proteolysis before secretion, thus supporting the formation of EC-SOD tetramers with variable binding capacity. The ECM-binding region contains a cysteine residue (Cys219) that is known to be involved in an intersubunit disulfide bridge. We have determined the redox potential of this disulfide bridge and show that both EC-SOD dimers and EC-SOD monomers are present within the intracellular space. The proteolytic processing of the ECM-binding region in vitro was modulated by the redox status of Cys219, allowing cleavage under reducing conditions only. When wild-type EC-SOD or the monomeric variant Cys219Ser was expressed in mammalian cells proteolysis did not occur. However, when cells were exposed to oxidative stress conditions, proteolytic processing was observed for wild-type EC-SOD but not for the Cys219Ser variant. Although the cellular response to oxidative stress is complex, our data suggest that proteolytic removal of the ECM-binding region is regulated by the intracellular generation of an EC-SOD monomer and that Cys219 plays an important role as a redox switch allowing the cellular machinery to secrete cleaved EC-SOD.
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